A Solution for Greater Biomass Utilization
Coal and nuclear power plants operate on a scale of hundreds of megawatts. Both capital and operating costs benefit from economies of scale. For waste biomass, the system would have to be scaled down to less than a megawatt to match the typical size of the biomass resource. High-pressure combustion boiler systems require certified operators, and a cost is associated with system maintenance and pollution control. Even with free fuel, the cost and difficulty of operating a small boiler system on a per-kilowatt electricity basis are often higher than the retail price of electricity from the grid.
The Centers for Renewable Energy and Biomass Utilization at the Energy & Environmental Research Center are investigating alternative technologies for converting solid biomass fuel to electricity at small scales. One such technology is small-scale gasification. Similar to a boiler, air is mixed with fuel to produce heat. However, this is where the similarities end. Where a boiler uses the heat to produce steam, a gasifier uses the heat to initiate chemical reactions to break down the biomass into its hydrogen and carbon monoxide constituents. The hydrogen and carbon monoxide can then be used in a fuel cell, gas turbine or internal combustion engine. This is a little trickier than simply burning the fuel.
Just enough air must be added to produce the necessary heat for the chemical reactions, but not so much that air ends up burning the hydrogen and carbon monoxide.
Small-scale gasifiers may have issues with high moisture contents in biomass, and small fractions of tars and oils can pass unconverted through the gasifier. These condensates can cause downtime and increased maintenance costs. To overcome these problems, researchers at the EERC have developed a process for thermally integrating a gasifier with the electricity converter, such as the fuel cell or gas turbine. Waste heat from the gas turbine or fuel cell is recycled back to the gasifier to help heat the gasifier. The addition of external heat to the gasifier allows for the use of higher moisture biomass. The gasifier can also be designed to physically increase the size of the gasification zone, minimizing the oils that can pass through unconverted. The goal is to decrease the cost of preparing the biomass for gasification and minimizing the oils leaving the gasifier.
Through funding from the Xcel Energy Renewable Development Fund, the EERC has constructed a bench-scale gasifier designed to be integrated with high-temperature fuel cells. Testing of the gasifier demonstrated exceptionally low tar and oil output with high-moisture wood. As part of the work, the gas output of the gasifier was used to power a small high-temperature fuel cell stack. The next step will be to scale the gasifier beyond the bench scale and integrate it with a larger electricity converter, a gas turbine. Using this concept, the EERC plans to thermally integrate a small gas turbine with a gasifier to put power onto the electrical grid. The goal of the project is to produce a low-cost, low-maintenance distributed power system for converting biomass to electricity. In addition to the design and testing of the power system, this project will quantify operating and manufacturing costs, and provide a commercialization road map to minimize time to market.
Phillip Hutton is a research manager at the EERC in Grand Forks, N.D. He can be reached at firstname.lastname@example.org or (701) 777-5204.